Two-dimensional spatial light modulators are widely used in a range of imaging applications for projection of color images. Because they form a complete, two-dimensional image at one time without requiring mechanical movement, spatial light modulators offer a number of advantages over other types of imaging devices, such as scanning lasers, for example.
A spatial light modulator can be considered essentially as a one- or two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate light, in cooperation with its support optics, by transmitting, blocking transmission, or reflection of incident light from a light source.
There are two salient types of spatial light modulators that are employed for forming images in projection apparatus. The first type, the liquid crystal device (LCD), modulates an incident beam by selectively altering the polarization of light for each pixel. An LCD may be transmissive, operating by selectively transmitting the incident beam through individual array elements, or reflective, selectively changing the polarization of a reflected beam at individual array elements. The second basic type of spatial light modulator currently in use is the digital micromirror device (DMD), disclosed in U.S. Pat. No. 5,061,049. The DMD modulates using time-phased reflection at each individual pixel site.
Projection apparatus using spatial light modulators include those disclosed in U.S. Pat. Nos. 5,325,137 and 5,743,610. A device of this type used for miniaturized image display, such as mounted within a helmet or supported by eyewear, is disclosed in U.S. Pat. No. 5,808,800.
Taking advantage of continuing development of spatial light modulators and of their support optical and electronic components, designers have been able to enhance the performance and reduce the cost of color projection apparatus. However, it can be readily appreciated that there is still room for improvement. It can also be readily acknowledged that improvements in performance and lower cost can help to make some of the potential benefits of digital imaging available where conventional optical imaging methods are primarily used.
One area where digital imaging affords great potential advantages is in diagnostic imaging, particularly for X-rays. Conventionally, X-rays and other types of diagnostic images are produced primarily on film. This is true even though many types of diagnostic images, such as X-rays, are initially obtained as scanned digital data. While some electronic display devices have been employed for X-ray viewing, these devices have, by and large, proved to be bulky and costly and have not provided the level of image quality necessary for wide acceptance.
A subtler problem also affects acceptance by medical professionals of electronically displayed X-rays as digital images. That is, the conventional X-ray is monochromatic, with images represented in varying tone or density values of a single hue. For example, grayscale images appear to be essentially monochromatic, using tone values from white to black. Some doctors and technicians, well acquainted with the color hue and tone quality provided by conventional X-ray development, are understandably reluctant to adapt to digital representation. Earlier displays of digital diagnostic images did not have the ability to emulate the familiar color hues and tones of conventional film-based images, and thus did not enjoy the necessary level of confidence in their accuracy, repeatability, and reliability.
The emphasis in design of digital projection systems, meanwhile, has consistently been directed to color projection. Monochrome display projection in general, such as grayscale projection, excites very little interest for electronic projection systems designers. Thus, while digital projector development has moved toward improved representation of color, the needs of diagnostic imaging remain fixed in a monochrome mode. Moreover, development apparatus for sensitized media used in diagnostic imaging, such as X-ray film, are known to provide different color hues, with typical base colors ranging from sepia to dark blue, depending on the system or lab processing method used to develop the media.
Spatial light modulators have been used in digital light box applications, as is disclosed in U.S. Pat. No. 6,246,450. However, in the apparatus described in U.S. Pat. No. 6,246,450, the light modulator is used only as a mask for controlling the areas of light provided for light box illumination, rather than directly as an image-forming source.
It is instructive to note that LCDs exhibit wavelength dependencies, by which modulation characteristics vary somewhat, depending on the wavelength of the incident beam. Thus, LCDs are not well-suited to white-light illumination, but provide a tint which is often considered to be objectionable. In practice, a projector employing LCDs either uses a separate color channel with a separate LCD for each color light component, typically red, green, and blue (RGB) or time-shares a single LCD, multiplexing each component light source. For each color channel, different LCD bias voltages may be used or different support filters or other optical components provided in order to compensate for wavelength dependencies. By comparison, the DMD is less sensitive to wavelength.
The need for handling separate colors, then, complicates the task of providing monochrome images, particularly where user preferences for particular color hues would be a factor, such as with conventional X-rays. Thus, it can be appreciated that there is a long-felt need for a projection system for diagnostic imaging that provides a suitable monochrome tone scale response for which the hue can be adjusted to observer preference and that provides a display with sufficient performance, reliability, and accuracy, at a cost that competes with that of existing X-ray film development and display systems.